Nest microclimate predicts bill growth in the Adelaide rosella (Aves: Psittaculidae)

Diet and foraging have traditionally been considered key drivers of bill morphology. It is now known that bills play an important thermoregulatory role, and recent studies revealed that temperature is positively associated with the size of bills relative to body size or weight in adult birds, in accordance with Allen's rule. Studies have attributed these patterns to local adaptation or an evolutionary response to climate change, but the contribution of ontogenetic plasticity remains unclear. We tested whether temperature experienced inside the nest predicted nestling growth in bill size and weight and in a parrot, the Adelaide rosella (Platycercus elegans adelaidae). We predicted that nest microclimate may affect bill ontogeny, leading to a positive association between relative bill size and temperatures during rearing. Growth in bill surface area was greater in nests that were warmer during the day and night, but temperature variability had no effect. Higher day and night-time mean temperatures, and less variable night-time temperatures, were positively associated with nestling weight. Our findings indicate that nest microclimate influences nestling growth, including relative bill size, and that daytime heat dissipation may be a driver of bill ontogeny. Bill plasticity in response to temperature during rearing could be an important but little studied contributor to morphology, due to the role of the bill in thermoregulation.